This invention relates generally to three dimensional coordinate measuring machines (or CMM""s). More particularly, this invention relates to a new and improved three dimensional CMM which is portable and provides improved accuracy and ease of use; and the application of this CMM to a novel method for programming the tool path of a multi-axis machine tool or robot.
It will be appreciated that everything in the physical world occupies volume or space. Position in a space may be defined by length, width and height which, in engineering terms, is often called an X, Y, Z coordinate. The X, Y, Z numbers represent the dimensions of length, width and height or three dimensions. Three-dimensional objects are described in terms of position and orientation; that is, not just where an object is but in what direction it points. The orientation of an object in space can be defined by the position of three points on the object. Orientation can also be described by the angles of alignment of the object in space. The X, Y, and Z coordinates can be most simply measured by three linear scales. In other words, if you lay a scale along the length, width and height of a space, you can measure the position of a point in the space.
Presently, coordinate measurement machines or CMM""s measure objects in a space using three linear scales. These-devices are typically non-portable, expensive and limited in the size or volume that can be easily measured.
FARO Technologies, Inc. of Lake Mary, Fla. (the assignee of the present invention) has successfully produced a series of electrogoniometer-type digitizing devices for the medical field. In particular, FARO Technologies, Inc. has produced systems for skeletal analysis known as METRECOM(copyright) (also known as Faro Arms(copyright)) and systems for use in surgical applications known as SURGICOM(trademark). Electrogoniometer-type devices of the type embodied in the METRECOM and SURGICOM systems are disclosed in U.S. Pat. No. 4,670,851 and U.S. application Ser. Nos. 593,469 filed Oct. 2, 1990 and 562,213 filed Jul. 31, 1990 all of which are assigned to the assignee hereof and incorporated herein by reference.
While well suited for their intended purposes, the METRECOM and SURGICOM electrogoniometer-type digitizing systems are not well suited for general industrial applications where three dimensional measurements of parts and assemblies are often required. Therefore, there is a continuing need for improved, accurate and low cost CMM""s for industrial and related applications.
A serious limitation in the practical usage of CNC or computer numerically controlled devices such as robotics and 5-axis machine centers is the time and effort required to program intricate and convoluted paths prior to performing typical robotic functions (such as welding or sanding) and/or typical machine tool functions (such as machining complex molded parts). Presently, this programming process entails a careful and meticulous step-by-step simulation based on trial and error.
The above-discussed and other problems and deficiencies of the prior art are overcome or alleviated by the three dimensional measuring instrument (e.g., electrogoniometer) of the present invention; and method of using the same. In accordance with the present invention, a novel, portable coordinate measuring machine comprises a multijointed (preferably six joints) manually positionable measuring arm for accurately and easily measuring a volume, which in a preferred embodiment, comprises a sphere preferably ranging from six to eight feet in diameter (but which may also cover diameters more or less than this range) and a measuring accuracy of preferably 2 Sigma+/xe2x88x920.0005 inch (and optimally 2 Sigma+/xe2x88x920.001 inch). In addition to the measuring arm, the present invention employs a controller (or serial box) which acts as the electronic interface between the arm and a host computer.
The mechanical measuring arm used in the CMM of this invention is generally comprised of a plurality of transfer housings (with each transfer housing comprising a joint and defining one degree of rotational freedom) and extension members attached to each other with adjacent transfer housings being disposed at right angles to define a movable arm preferably having five or six degrees of freedom. Each transfer housing includes measurement transducers and novel bearing arrangements. These novel bearing arrangements include prestressed (or preloaded) bearings formed of counter-positioned conical roller bearings and stiffening thrust bearings for high bending stiffness with low profile structure. In addition, each transfer casing includes visual and audio endstop indicators to protect against mechanical overload due to mechanical stressing.
The movable arm is attached to a base or post which includes (1) a temperature monitoring board for monitoring temperature stability; (2) an encoder mounting plate for universal encoder selection; (3) an EEPROM circuit board containing calibration and identification data so as to avoid unit mixup; and (4) a preamplifier board mounted near the encoder mounting plate for transmission of high amplified signals to a remote counter board in the controller.
As in the prior art METRECOM system, the transfer casings are modular permitting variable assembly configurations and the entire movable arm assembly is constructed of one material for ensuring consistent coefficient of thermal expansion (CTE). Similarly as in the METRECOM system, internal wire routing with rotation stops and wire coiling cavities permit complete enclosure of large numbers of wires. Also consistent with the prior art METRECOM system, this invention includes a spring counterbalanced and shock absorbed support mechanism for user comfort and a two switch (take/accept) data entry device for allowing high precision measurements with manual handling. Also, a generalized option of the type used in the prior art METRECOM system is provided for the measurement of variables in three dimensions (e.g., temperature may be measured in three dimensions using a thermocouple attached to the option port).
The use of a discrete microprocessor-based controller box is an important feature of this invention as it permits preprocessing of specific calculations without host level processing requirements. This is accomplished by mounting an intelligent preprocessor in the controller box which provides programmable adaptability and compatibility with a variety of external hosts (e.g., external computers). The serial box also provides intelligent multi-protocol evaluation and autos witching by sensing communication requirements from the host. For example, a host computer running software from one manufacturer will generate call requests of one form which are automatically sensed by the controller box. Still other features of the controller box include serial port communications for standardized long distance communications in a variety of industrial environments and novel analog-to-digital/digital counter boards for simultaneous capture of every encoder (located in the transfer housing) resulting in highly accurate measurements.
Efficient on-site calibration of the CMM of the present invention is improved through the use of a reference ball positioned at the base of the CMM to obviate potential mounting complications to system accuracy evaluation. In addition, the CMM of this invention includes means for performing a volumetric accuracy measurement protocol on an interim basis, preferably using a novel cone ballbar device.
In accordance with still another embodiment of this invention, a novel method is provided for programming the complex paths required for operations of robotics and multi-axis machine centers in the performance of typical functions such as sanding or welding (commonly associated with robotics) and machining molded parts (commonly associated with multi-axis machine tools). In accordance with this method, it is desired to replicate in a computer controlled machine, the operation or a path (defined both by direction and orientation) of an experienced human operator. This is accomplished using the CMM of this invention whereby the CMM operator uses the lightweight, easy-to-handle and passive electrogoniometric device described above with a simulated tool at its digitizer end and emulates either a desired tool path or manufacturing operation. As this path or operation is emulated, the position and orientation data (in both the X, Y and Z directions and/or I, J and K orientations) of the CMM is accumulated and stored. This data is then transferred using industry standard formats to a computer numerically controlled (CNC) device such as a robot or machining center for the reproduction of the motions emulated using the CMM. As a result, the computer controlled device has provided to it, in a quick and efficient manner, the exact path and/or operations data for performing a task regardless of the complexity involved. Prior to this method, the programming of such tasks involved meticulously and carefully programmed step-by-step sequences using simulation and trial and error.
To date, Robotic programming has operated principally through a process called teach mode. In the teach mode approach the robot is directed to perform and memorize a task. A technician will direct a robot through a controller panel and joy stick to perform the desired motions. The robot""s actions are stored as a series of stepwise motions including rotations of various joints and actions of specific end-effectors.
Because of the nature of this method the actual absolute dimensional position was not as important as the ability to repeat a position previously taught.
The industry has witnessed a significant increase in computerization in the design and manufacturing environments and an increase in the number of complex curved paths and types of end-effectors used such as laser. This means that robotic path data begins to resemble typical CAM (Computer Aided Manufacturing) data. Typical computer controlled machining centers are both dimensionally accurate and repeatable. This is not the case for the typical multi-jointed robot, for the reasons described above.
The above-discussed and other features and advantages of the present invention will be appreciated and understood by those skilled in the art from the following detailed description and drawings.